In recent years, light-emitting devices including light-emitting elements have been improved so as to have higher luminance and emit white light and have been used as backlights for cellular phones, large liquid-crystal televisions, and the like. In particular, glass ceramic sinter has been attracting attention because it has high reflectivity and, in addition, it has high heat resistance and high durability and does not deteriorate even under exposure to ultraviolet radiation for a long period of time, compared with synthetic resins having been used for existing substrates for mounting light-emitting elements.
For example, Patent Literature 1 discloses a glass ceramic sinter formed from, as ceramic raw materials, borosilicate glass, alumina, a group 2 element such as calcium, and metal niobium or niobium oxide so that it has high reflectivity in the visible-light range.